Integrated optics enables large-scale integration of optical components on a chip, and enables complex optical processing to be achieved, including enabling functionality that is not practicably achievable with bulk components. High-index-contrast (HIC) optical waveguides and resonators, having a large difference between the core and cladding refractive indices, allow components only a few wavelengths in size that have substantially low radiation losses. For example HIC, strongly-confined microring resonators support small bending radii (on the order of a few micrometers) with low radiation losses, thus permitting large free-spectral-range (FSR>20 nanometers (nm)) and high quality factor (Q>100,000) resonances that are important for frequency selective filters for multiple applications, including optical channel add-drop filters for wavelength-division-multiplexed (WDM) networks. Multiple frequency-aligned resonators may be combined into higher-order filters to provide more complex and selective filter response function shapes.
Wavelength-division-multiplexed (WDM) optical transmission systems carry multiple wavelength channels simultaneously on a single guiding optical line. Their large information capacity is useful in telecommunication applications, but also for intra-chip and inter-chip photonic networks for advanced high-performance microprocessors and systems for supercomputers; and for various high-bandwidth applications where electronic-photonic hybrid integrated circuits may offer significant advantages, such as high-resolution, high-sampling-rate analog-to-digital converters, voice and image data processing, and biological data processing that are well suited to data parallelism.
To offer the possibility of WDM signal processing, such as switching and routing, all in the optical domain on a chip, integrated optical circuits comprising various functional optical components need to employ waveguiding structures that can couple light efficiently to and from optical fibers.
Optical channel add-drop filters (CADFs) are one important class of functional optical components employed in integrated optical circuits. CADFs typically have an input port, at least a drop or add port, a through port, and preferably a further port which, in combination with the drop or add port, forms a pair of add and drop ports. CADFs support narrow passbands covering typically a single wavelength channel. They enable transmission of a signal in the selected wavelength channel within the passband from the input port to the drop port with low loss (preferably less than 3 dB), while suppressing crosstalk from signals in other wavelength channels in the operating wavelength range (OWR) of the filter (preferably by at least 30 dB). All channels outside the filter passband and in the OWR of the filter are transmitted from the input port to the through (or express) port, preferably with much less than 3 dB insertion loss. The selected wavelength channel within the filter passband is typically fully removed from the input spectrum with preferably over 30-40 dB extinction of the signal remaining in the through port. This high extinction helps prevent crosstalk with a new signal, incident at the add port, which is inserted into the through port at the selected channel wavelength. Wavelength channel passbands are typically 10-100 GHz wide and are typically spaced by 25-200 GHz as, for example, specified by the International Telecommunications Union (ITU) wavelength grid standards.
The OWR of a CADF is preferably a wide optical band, e.g., the C-band communication window of 1530-1570 nm.
Integrated optical filters with a single passband over a wide operating wavelength range can be made using optical resonators, for example microring resonators, with a large FSR equal to or larger than the OWR, such that only one resonance lies within the range. Large FSR resonators can be made by making the resonator small in size so that, in traveling-wave resonators like rings, the path length is short and spaces longitudinal resonances further apart spectrally. Small ring resonators have tight bend radii and optical radiation confined and propagating in such a ring tends to experience bending radiation loss, giving rise to a low quality factor, Q. Radiation loss can be reduced to an acceptable level by designing waveguides using high refractive index contrast (HIC) between the waveguide core and cladding, such as SiN (n˜2.2 near 1550 nm wavelength) or Si (n˜3.5) core, and silica (n˜1.45) or air (n˜1) cladding. In turn, HIC resonators are small and require fine lithography, can have significant propagation losses due to surface roughness, and their resonant frequency may be sensitive to small dimensional errors resulting in fabrication.
At the same time that HIC waveguides, resonators and interferometers provide important enabling features for CADFs and other integrated optical devices, they pose significant challenges. On the one hand, HIC provides strong optical confinement, thereby enabling small optical resonators with low radiation losses and thus high loss Q's. Small resonators lead to both high integration density, and large FSR. On the other hand, HIC waveguides provide significant fabrication challenges. One challenge is the requirement of finer lithographic resolution to realize the smaller features of HIC devices. More important is the challenge to combat the sensitivity of HIC structures to dimensional and index variations. For example, to make a microring resonator with a polarization-independent resonance frequency would require atomic-scale dimensional control.
Preferably, during the dynamic reconfiguration of optical components such as reconfigurable optical add-drop multiplexers (R-OADMs), i.e., of their add-drop filters, that operate on a subset of the WDM spectrum, the data flow on other express wavelength channels in the through port is not interrupted or deteriorated (e.g., by insertion loss or dispersion) during the reconfiguration operation. This is referred to as hitless switching or hitless reconfiguration of the optical component. Some waveguide designs for hitless switchable integrated-optical filters require use of a combination of resonators and interferometers. In such cases the sensitivity of interferometer components is equally critical.
In general, resonance frequency sensitivity in multiple-resonator filters is important when it is desired to have the filter frequency aligned without post-fabrication trimming or tuning. It is possible, in principle, to apply tuning elements to individual resonators, such as one heater per resonator where thermo-optic index tuning is employed. However, it is still important to align the resonance frequencies prior to actuation of tuning elements. This is because the tuning range of the device may be reduced if a part of the tuning range of various tuning elements is used to first compensate for resonance misalignment between resonant cavities due to fabrication sensitivities and errors.
A further important concern in HIC is the sensitivity of the propagating mode to surface roughness on the waveguide core and any other layers seen by the optical mode. HIC generally enhances sensitivity because a high index perturbation more strongly scatters light than a low index perturbation. As a result, propagation loss may result from sidewall and top/bottom-wall roughness in HIC waveguides. Sidewall roughness is typically determined by lithography and etching processes, and tends to be much larger than the top/bottom-wall roughness of an HIC waveguide, made by a typical planar fabrication process based on lithography. There are several other possible sources of loss in the HIC core material (or in the cladding material, which is relevant only in the spatial region where the optical mode has substantial intensity). These sources may include material absorption, and bulk scattering such as scattering from spatial index non-uniformities (that may be due to density variation) or Rayleigh scattering from a crystal lattice. Crystalline core or cladding materials typically have negligible intrinsic absorption for wavelengths with a photon energy below the bandgap energy of the material. For silicon, an important core material, the intrinsic absorption is negligible in the 1500-1600 nm wavelength region used for telecom applications, as well as generally at wavelengths longer than the bandgap wavelength around 1100 nm. Non-ideal crystalline structure or dangling bonds have been found to cause absorption. Methods have been published in integrated optics literature to permit passivation that substantially reduces absorption centers. On the other hand, loss due to Rayleigh scattering from the lattice of an ideal crystalline material is small in comparison to the sidewall and top/bottom-wall roughness loss. Therefore it is expected that the waveguide propagation loss will ultimately be limited by sidewall and top/bottom-wall roughness.
Generally, the prior art describes waveguides with square and near-square (up to about 2:1 aspect ratio) core-region cross-sections. This is because it is generally recognized in the art of designing waveguides as desirable to aim as a design goal for the maximum possible strength of optical confinement (i.e., maximum effective index of the guided mode), and in many cases for polarization independent operation. Strong confinement in waveguides permits small bending radius while guaranteeing substantially low bending radiation loss, which in turn enables the design of small bends, and ring resonators with large FSR and low loss, i.e., high loss Q (e.g., >10,000 to 100,000). Strongest confinement is generally known to be provided in square or near square waveguides (up to about 2:1 aspect ratio in index contrasts approximately up to 2.5:1), since, given the constraint of maintaining e.g., single TE mode operation, the majority of the optical field is contained in the core in such cross-sections, thus giving maximal effective index (and thus maximal confinement). A further constraint in high-index contrast waveguide design has been the available thicknesses of core materials. Silicon-on-insulator (Si core index 3.5) wafers are typically available with a 200 nm Si layer so nearly all Si waveguide designs are about 200 nm to 250 nm thick.
Stoichiometric silicon nitride (Si3N4 index 2.0) is a second material used for waveguides with moderate index contrast. It is well known in current literature that the core layer thicknesses of Si3N4 that can be grown are limited by stress build-up to about 300 nm, as thicker layers may crack due to stress. This is the reason why most Si3N4 waveguides are typically no more than 200-300 nm thick. See, e.g., N. Daldosso, et al., “Fabrication and optical characterization of thin two-dimensional Si3N4 waveguides,” Materials Science in Semiconductor Processing 7 (2004) pp. 453-458; N. Daldosso et al., “Comparison Among Various Si3N4 Waveguide Geometries Grown Within a CMOS Fabrication Pilot Line,” Journal of Lightwave Technology, Vol. 22, No. 7 (July 2004) pp. 1734-1740; and M. Melchiorri, et al., “Propagation losses of silicon nitride waveguides in the near infrared range,” Appl. Phys. Lett. 86, 121111 (2005), all of which are incorporated by reference herein. Silicon-rich SiN (with about 2.2 index) has also been used as a core material. Since it can be grown in thick layers (400 nm and higher), the aspect ratios used in this system are about 2:1 or smaller. See Popovic, M. et al., “Multistage high-order microring-resonator add-drop filters,” Optics Letters, Vol. 31, No. 17 (Sep. 1, 2006) pp. 2571-2573; M. A. Popovic, M. R. Watts, T. Barwicz, P. T. Rakich, L. Socci, E. P. Ippen, F. X. Kartner and H. I. Smith, “High-index-contrast, wide-FSR microring-resonator filter design and realization with frequency-shift compensation,” in Optical Fiber Communication Conference (OFC/NFOEC) Technical Digest (Optical Society of America, Washington, D.C., Mar. 6-11, 2005), paper OFK1, vol. 5, pp. 213-215, both of which are incorporated by reference herein. Particularly in doped-silica and semiconductor ridge waveguides, attempts to provide polarization independent operation, i.e., identical effective indices for the fundamental TE and TM modes, also lead one to consider square or near-square waveguides. See B. E. Little, et al., “Very high-order microring resonator filters for WDM applications,” IEEE Photonics Technology Letters, Vol. 16, No. 10 (October 2004) pp. 2263-2265; and Chan, S. P. et al., “Single-mode and polarization-independent silicon-on-insulator waveguides with small cross section,” Journal of Lightwave Technology, Vol. 23, No. 6 (June 2005) pp. 2103-2111, both of which are incroprated by reference herein.
Previous literature also considers waveguides with aspect ratios larger than 2:1. See, e.g., Baehr-Jones, T., et al., “High-Q ring Resonators in Thin Silicon-on-Insulator,” Applied Physics Letters, Vol. 85, No. 16 (18 Oct. 2004) pp. 3346-3347; And Guo, J., et al., “Characterization of Si3N4/SiO2 planar lightwave circuits and ring resonators,” Proceedings of SPIE, Vol. 5350 (2004) pp. 13-22, both of which are incorporated by reference herein. In high index contrast (2.5:1), waveguides with about 4:1 aspect ratio in Si were made in order to reduce confinement of the optical mode in the core and force more of the mode field out into the cladding, so that it may see less absorption in the core, where the core material may have absorption. In lower index contrast (about 1:4), waveguides with 6:1 aspect ratio in Si3N4 were made in order to increase confinement, while maintaining single-mode operation, because the thickness could not be increased due to the material-stress-caused maximum thickness constraint. The motivation for increasing width relative to height in these cases is to increase the optical confinement by increasing area, under a core thickness constraint, or to expel the field from the core, by decreasing thickness, and thus reduce confinement.
It is important to develop waveguide designs that have low sensitivity to fabrication errors and that accumulate minimal propagation loss from a given surface roughness of the waveguide.